Working together

for a safer world

Maximising value from legacy raster bathymetry datasets

Gareth Grewcock

@ gordy99

Rasters Revealed 2017

21.02.2017

Bathymetry = 'submarine topography'

Acquistion = £££'s

marine DTM

New Bathymetry Survey @ ~£20,000 per day?

Raster Bathymetry DTM = not fit for engineering purposes 

-  excessively smoothed and delivered as a 5x5m Grid.

 - fairly coarse for shallow bathymetry survey

Key seabed features lost

Let's review the raw survey data, does it support a higher resolution DTM?

Why has it been delivered at 5x5m?

Discrepancy of absolute depths = tidal issue 

oversmoothed bathymetry surface 5x5m

VORF control

surface 1x1m

Raw data reprocessed

1x1m

The raw data does support 1x1m

If we can fix the tidal issue =

130 NW - SE reprocessed lines

30 VORF control lines X 130 reprocessed lines  = 3600 tidal correction points

5x5m Oversmoothed DTM

VORF control DTM 1x1m

30 Tidal Control Points per reprocessed survey line

intersections

calculate the mean difference at each intersection

We can create a raster correction surface using the tidal correction points (mean differences) to level each 1x1m reprocessed survey line to a common level

Effectively, we're dealing with XYZ data

Finally - lets talk raster processing...

manual reptitive task = automated solution

{Code Snippets}

How did we create the tidal correction raster surface?

automated workflow

task #1

syntax

grdmath rstr-a.tif rstr-b.tif SUB = output.tif=gd:GTiff

Difference each (130) reprocessed 1x1m survey line from the VORF control surface

VORF Control Lines (1 DTM)

Reprocessed 1x1m survey line

task #2

output from task #1

gdalwarp -cutline output.shp -csql "SELECT * FROM output WHERE ID = '29'" -crop_to_cutline out-clip.tif

syntax

Create vector extent for each NE-SW VORF control survey line

grdmath rstr-a.tif=gd:GTiff -10000 GT = output.tif=gd:GTiff

gdal_polygonize output.tif -f "ESRI Shapefile" output.shp

Clip the difference output by each (30) NE-SW VORF control survey line

automated workflow

task #3

output from task #2

from osgeo import gdal

clip_out = r"F:\Temp\clip-out.tif"

src_ds = gdal.Open(clip_out)

if src_ds is None:
                <font></font>
    print 'Unable to open INPUT.tif'
                <font></font>
    sys.exit(1)

srcband = src_ds.GetRasterBand(1)
stats = srcband.GetStatistics( True, True )
            

x, y

GDAL Windows Python Bindings | http://www.gisinternals.com/release.php

Calculate the tidal correction offset value

DTM centre coordinates

- min Z value
- max Z value
- mean Z value
- standard deviation

automated workflow

Mean value

(tidal correction offset)

Pipe out to next stage..

automated workflow

Calculate the tidal correction offset value

30 Tidal Correction Offsets for 1x1m reprocessed survey line

task #4

natural neighbour interpolation

syntax

ArcPy

#confession

automated workflow

task #5

Level raw legacy survey line with the tidal correction raster surface

automated workflow

Reprocessed 1x1m survey line

Tidal correction raster surface

=

A tidally corrected, 1x1m levelled survey line

task #6

Create final 1x1m levelled raster DTM surface

directory of 130 levelled survey lines

automated workflow

• Levelled all 130 1x1m reprocessed survey lines
• 1x1m DTM - now fit for engineering purposes
• Seabed features now clearly identified

results

Ultimately - raster processing saved the client $$$'s!!

Working together

for a safer world

Gareth Grewcock

Geospatial Team

Survey & GeoEngineering

Lloyd's Register

T +44 (0)1225 485800  E

Lloyd’s Register and variants of it are trading names of Lloyd’s Register Group Limited, its subsidiaries and affiliates.
Copyright © Lloyd’s Register. 2016. A member of the Lloyd’s Register group.

Gareth.Grewcock@lr.org